when、while、as引导时间状语从句的区别
这三个词的意思很简单，都有“当……时候”的意思。但学生经常会问三个词的区别在哪儿，特别是在做选择题的时候。别说是学生，就我个人而言，做这样的选择题要保证百分之百的正确也是不可能的。现根据大量的实例和个人的思考，做一点小结，供大家参考。
一、when 的用法
如果只从现象来看，when 从句用的最多的是一般过去时，而主句的时态没有限制，根据具体情况而定。
When he was a child he was always trying out new ideas.
他小时候就常常试验一些新的设想。
when she came into my room I was just reading a book.
她走进我房间时，我正在看书。
Were you writing when the teacher came in?
老师进来的时候，你在写信吗？
Sorry，I was out when you called me.
对不起，你打电话来的时候我出去了。
He was on the point of leaving when someone knocked at the door.
他正要走,这时有人敲门。
I thought of it just when you opened your mouth.
就在你要说话的时候,我也想到了。
I had hardly[scarcely] closed my eyes when someone knocked at the door.
我刚一闭上眼，就有人在敲门了。
根据以上的例句，我们可以总结出一点：when 从句的A事件，相当于另一个事件B发生的时间点。也就是说，when 从句的重点不在动作本身发生的状态，而只是把它作为一个时间点，所以when 多数情况下用的是一般过去时，则不用正在进行时。因为如果用正在进行时，它表示的就是一段时间而不是一个时间点了。根据这一点，有的文章补充说：when 从句的动词大多是瞬时动词。这种说法也可以参照。
实际上，when 从句也可以有其它的时态，但几乎也不用进行时，因为它也只是作为一个时间参照点。例如：
When I got to the airport，the guests had left.
当我赶到飞机场时，客人们已经离开了。
When he had finished his homework，he took a short rest．
当他完成作业后，他休息了一会儿。
Why do you want a new job when you have got such a good one already？
你已经找到如此好的工作，为何还想再找新的？
You shall borrow the book when I have finished reading it.
在我读完这本书后，你可以借阅。
When the manager comes here for a visit next week，Ill talk with him about this. 下周，经理来这参观时，我会和他谈谈此事。
二、while 的用法
相比于when 来说，while 从句的侧重点就不一样了。while 从句的侧重点在于描述动作正在发生的状态，它的意思是：当while 事件正在发生的时候，另一件事如何如何。所以，while 从句一般用的是正在进行时。而另一件事的状态没有硬性的要求，根据具体情况而定。例如：
While my wife was reading the newspaper, I was watching TV.
当妻子正在看报纸的时候，我正在看电视。
While Jim was mending his bike, Lin Tao came to see him.
正当吉姆修自行车时，林涛来看他。
While they were talking , the bell rang.
正在他们谈话的时候，上课铃响了。
You can't do your homework while you're watching TV.
你不能一边看电视一边做家庭作业。
While John was sitting biting his nails, I was working out a plan to get us home. 约翰坐在那里咬指甲时,我想出了一个回家的办法。
从时间的角度来看，while 表示的是一段时间，是一个过程。这是while 的侧重点。因此，如果含有“一段时间”的含义的时候，就可以用while。例如：
Strike while the iron is hot. 趁热打铁。
这句话中，是说趁着铁是热的这段时间，赶紧打铁。如果换成 when 意思就变了，相当于说铁只热了一下，打一下，然后铁就冷了。这显然不符合文意。再例：
—I'm going to the post office.
—While you're there, can you get me some stamps?
三、as 的用法
1、as 从句表示的也是一件事情正在发生，另一件事也正在进行当中。但与 while 从句不同的是，as 从句用的一般不用正在进行时，而只是一般过去时。as 从句一般可以翻译成“边……边……”。例如：
As my mother sang those old songs, tears ran down her cheeks.
当我妈妈唱起那些老歌时，眼泪顺着她的脸颊流了下来。
The students took notes as they listened．
学生们边听课边做笔记。
As we talked on, he got more and more excited.
我们继续往下谈的时候,他越来越兴奋。
The little girls sang as they went．
小姑娘们一边走，一边唱。
Just as he caught the ball, there was a tearing sound.
当他抓住球的时候,有一种撕裂的声音。
2、as 表达的事件，往往只是主句动作发生的背景或条件时，as 只是一个次要的时间说明，不像while 从句有强调 while 动作本身的意思。因此，as 常常翻译成“随着……”之意。 例如：
As the time went on，the weather got worse.
随着时间的推移，气候更加糟糕。
The atmosphere gets thinner and thinner as the height increases.
随着高度的增加，大气越来越稀薄。
As years go by，China is getting stronger and richer.
随着时间一年一年过去，中国变得越来越富强了。
3、少数情况下，如果强调动作正在发生，as 从句也可以用正在进行时。这只能算是特例了。
The sad mother sat on the roadside，shouting as she was crying.伤心的妈妈坐在路边，边哭边叫。
As we was going out, it began to snow.
我们出门时，开始下雪了。
He came in as I was going to bed.
正要上床睡觉，他进来了。
四、when, while, as 的互换
如果从句动作和主句动作同时发生，并且从句动作为延续性动词时，when，while，as 可以互换使用。这种情况下，它们的细微区别恐怕连英、美人自己也说不清了。
When /While /As we were dancing，a stranger came in.
当我们跳舞时，一位陌生人走了进来。 [dance 为延续性动词]
When /While /As she was making a phonecall，I was writing a letter.
当她在打电话时，我正在写信。 [make为延续性动词]
While/When/ As I was walking down the street, I noticed a police car in front of the bank.
我顺着马路往前走时,发现银行门前停着一辆警车。
[附] 常用的表示时间的关联词
常用关联词：when, as, while, as soon as, while, before, after, since , once,,till, until 等。
特殊关联词：the minute, the moment, the second, every time, the day，the instant, immediately , directly, instantly, no sooner … than, hardly …when, scarcely … when 等。
例句：
I didn't realize how special my mother was until I became an adult.
The children ran away from the orchard the moment they saw the guard.
No sooner had I arrived home than it began to rain.
Every time I listen to your advice, I get into trouble.